Process of drying sewage sludge

ABSTRACT

A novel process is disclosed for the transfer of heat and/or matter between two or more gaseous or liquid agents, the process utilizing a carrier substance continuously circulated in closed cycle successively through at least two working spaces wherein heat and/or mater is respectively taken up and yielded. The carrier substance is present in a thin layer and the agents are caused to flow across the layer of carrier substance at both working spaces. Novel mechanical assemblies for the performance of said process are also disclosed herein.

United States Patent 1 1 3,619,422

[72] Inventors Roman Rummel, deceased [56] References Cited i i H B m Id UNITED STATES PATENTS sa umme en, ru rmgar Rummel, Heir, Backnang;Heinz Rummely 1,614,387 1 1927 Pereda 165/107 Heir Bruhblnge Rummel HeirBruhl all 2,723,954 11/1955 Yung..... 34/9X of crman 3,272,335 9/1966Nettel 210/68 y 3,290,790 12/1966 Kunii et a1. 34/9 [21] Appl. No.758,185

3,306,236 2/1967 Campbell 1 /8 3 319 587 5 1967 Alb 1 110/8 PatentedNov. 9, 7 1 1 el'tSOl'l eta [73] Assignee Von Roll AG PrimaryExaminer-Michael Rogers Gerlaflngen, Switzerland Att0rneys-Jacobi andDavison and Werner W. Kleeman 1321 Priority Sept. 4, 1967 [331Switzerland [31 12321/67 ABSTRACT: A novel process is disclosed for thetransfer of t heat and/or matter between two or more gaseous or liquid 5,1 1 OF Y G SEWAGE QLUDCE agents, the process ut1l1z1ng a carriersubstance continuously aims Drawmg Flgs' circulated in closed cyclesuccessively through at least two [521 U.S. Cl 210/10, working spaceswherein heat and/or mater is respectively 34/9, 1 10/8, 165/107, 210/67,210/68, 210/152 taken up and yielded. The carrier substance is presentin a thin [5 l I Int. Cl C02c 3/00 layer and the agents are caused toflow across the layer of car- Field of Search 260/683; rier substance atboth working spaces. Novel mechanical as- 165/104, 106, 107; 1 10/8, 15;34/9, 10, 57; 210/2, semblies for the performance of said process arealso disclosed 10, 12,68, 67, 152 herein.

PAIENTEUNM 9 Ian SHEET 2 BF 4 PROCESS OF DRYING SEWAGE SLUDGE BACKGROUNDOF THE INVENTION The present invention relates to both a process and toa mechanical assembly for the transfer of heat and/or matter between twogaseous or liquid agents, by means of a minute and loose, solid carriersubstance in continuous circulation through at least two working spacesin succession, the carrier substance taking up and respectively yieldingheat and/or matter from and to one of the aforesaid agents.

Heat exchangers using a minute and loose carrier substance in a closedcircuit configuration are already know, the closed circuit usuallycomprising vertical chambers or pits, wherein the carrier substance isheated in counterflow by a first gaseous or liquid agent, and thereupontransfers the heat in a second similar device to another agent also incounterflow with the carrier substance. The primary advantage of suchheat exchangers resides in the wide choice available for the carriersubstance, whereby a substance can be chosen to best meet the respectiverequirements. Thus, finer or coarser carrier substances may be adopted,presenting'a larger or smaller active surface in contact with the givingor receiving agent. Furthermore, carrier substances of mineral ororganic nature may be used, to best withstand the physical and/orchemical phenomena and reactions liable to occur, such as corrosion,erosion, temperature shocks, and the like. Finally, the carriersubstance may act as catalyst for the chemical reactions expected or bea support for the catalyst proper.

In comparison, heat exchangers of the recuperative and regenerative typeare far less versatile.

On the other hand, heat exchangers having recirculated carriersubstances are obviously more costly in operation, due to the energyconsumed, but this disadvantage is by far outweighed by the functionaladvantages discussed above. Furthermore, regenerative heat exchangersare very bulky and the necessary switchover mechanisms are very costly.Because of the large active surface of the carrier substance, heatexchangers of the type described herein have a high specific capacity inrelation to their volume; far above that of recuperators andregenerators.

In spite of their multiple advantages, heat exchangers with recirculatedcarrier substances are not widely used. The reason apparently lies inthe fact'that for high efficiency the carrier substance must be presentin comparatively thick layers, to ensure a homogeneous repartition ofthe agents flow through it. In the case of pits with circular crosssection, actual practice has shown that the height of the active layermust be at least equal to'its diameter. The great thickness of the layercauses a large drop of pressure-head to the stream of the agent, whichis most cases is prohibitive. Thus the use of heat exchangers withrecirculated carrier substances has so far been limited to such caseswhere the drop of pressure-head in the layer could be accepted.

From the functional point of view, the transfer of matter to and from acarrier substance, by known processes, such as absorption, adsorption,chemical binding and the like, is quite similar to the exchange of heat.Accordingly, the following considerations and the embodiments to bedescribed below, although referring to the exchange of heat, are equallyapplicable for the transfer of matter.

As is well known, in the case of a fluid flowing through a layer ofminute and loose matter, the drop of pressure-head in the layer is alinear function of the length of path, and a square function of thevelocity. When considering a layer of constant volume, it is thereforeobvious that by increasing the extension of the layer and simultaneouslydecreasing the length of path, i.e. the thickness of the layer, anydesired reduction of the drop of pressure-head is at least theoreticallypossible. Limitations, however, arise, since in practice it is notpossible to achieve a homogeneous flow of a gaseous or liquid agentthrough a rather thin layer of great extension. This also is true forboth processes of transferring heat and/or matter, and, of course, fortheir combination as well.

The high drop of pressure-head in the layer of carrier substance is,therefore, the primary and decisive hindrance to a wider utilization ofthe method based on recirculated carrier substances for the transfer ofheat and/or matter.

SUMMARY OF THE INVENTION It is a primary object of the invention toovercome and remedy the above described inconveniences of prior art heatand/or matter transfer methods using recirculated carrier substances. Itis a further, more specific object of the invention to reduce, in asimple way, the previously prohibitive high drop of pressure-head in thelayer of carrier substance, while, at the same time, maintaining theadvantages offered.

To this effect, the present invention primarilyis directed to a novelprocess of transfer of heat and/or matter, using a solid carriersubstance of minute and loose constitution, which is circulated inclosed circuit through two working spaces in succession, to take up heatand/or matter in the first working space and yield heat and/or matter-inthesecond one, from and respectively to a stream of a gaseous or liquidagent, or vice versa.

In accordance with the invention, the novel process is characterized bythe features that, in both working spaces, the carrier substance ispresent in a thin layer and that, under the effectof gravity, thecarrier substance is moved downwards from one working space into theother one at an uniform rate under control, and that the streams ofagents in both working spaces are caused to flow across the moving layerof carrier substance.

Another object of the invention is the provision of a mechanicalapparatus of novel construction for the performance of the processoutlined above, the apparatus or assembly being characterized by thefeatures that the-working spaces are shaped as narrow and slender pits,with their sidewalls permeable to the stream of the agent involved withinlet and outletopenings for said agent, and that at least oneadjustable device is provided for controlling the rate of recirculationof the carrier substance through the working spaces.

In a preferred embodiment of the invention, the two working spaces mayeach be divided-in at least two sections, connected by conduits outsidethe permeable sidewalls, and arranged in succession relative to theagents flow. With this construction, combinations of crossandcounterflow are possible, by means of which the temperature of theagents, when leaving the last section of the respective working space,is practically the same as in pure counterflow. In the case of transferof matter, the above considerations still apply, as the referencesto'temperature would be replaced by considerations of concentration.

In another preferred embodiment of the invention, the permeablesidewalls of the working-spaces may consist either of a porous material,or of a trellis tube, or of oblique plates arranged as in Venetianblinds.

In still another preferred embodiment of the invention, the two workingspaces are coaxially arranged and connected by an equally coaxialchannel of adequate length. At workingpressures not too different in thetwo working spaces, the connecting channel which is filled with thecarrier substance, presents a sufficiently high resistance to the flowof the agents so that a practical separation of the working spaces isachieved, thereby avoiding the flow of the agents from one working spaceinto the other, or vice versa.

In yet another preferred embodiment of the invention, the working spacesare connected by channels, evenly distributed over their cross section,to serve as passage for the carrier substance from one working space toanother, the channels being of sufficient length and of sufficientlynarrow cross section to ensure practical separation of the workingspaces with respect to the agents therein, as discussed above.

In a preferred modification of the last two embodiments, partialopenings are provided in the connectingchannel as an inlet for anoccluding agent such as an inert gas or steam,

which may flow into one or both working spaces without influencing theprocess of transfer of heat and/or matter.

In still another preferred embodiment of the invention, a plurality ofworking spaces may be provided for several processes of differentnature.

In another preferred embodiment of the invention, one of the two workingspaces may have impermeable walls, with openings at the upper and lowerends, as inlet and outlet for the respective agent, which thereby may bedirected in parallel or counterflow with the carrier substance. Thecombination of this with the the agents normal crossflow may be usefulin some cases.

In a further preferred embodiment of the invention, pits of annularcross section may be used as working spaces, which leads to an assemblyof overall cylindrical form, which may be advantageous due to itscompactness and solidity.

When used for preheating the combustion air in a steam boiler plant, oneof the working spaces may be incorporated in a flue duct to allow thehot combustion gases to heat the carrier substance.

In a preferred embodiment of the invention used for drying sewage sludgein combination with a refuse incinerator, metallic balls may be used asthe carrier substance, to be heated in the upper working space bycombustion gases and thereupon to be sprayed with liquid sewage sludgein the lower working space. The water vapor emanating from the lowerworking space is then caused to comingle with the stream of flue-gaseffluent from the incinerator, while the dried sewage sludge isdischarged together with the carrier substance and separated from it bysieving, before the carrier substance is recirculated into the upperworking space. The separated dry sewage sludge may subsequently be fedinto the furnace of the incinerator. In order to avoid possibleadherence of the dry sewage sludge to the metallic carrier substance,the second working space may preferably be a rotating kiln, wherein thestill liquid sewage sludge is sprayed.

In another preferred embodiment of the invention, hydrocarbons may becracked, by means of a catalyst contained in the carrier substance. In afirst working space, hot combustion gases drawn from a conventionalfurnace are used for heating up the carrier substance, while in a secondworking space, a stream of gaseous or vaporized hydrocarbons is broughtinto contact with the hot carrier substance to be heated up and cracked,the cracking products being then cooled and submitted to adequatetreatment. The carrier substance is then passed into a third workingspace, therein to heat the combustion air used in the furnace, and iseventually recirculated into the first working space to complete thecircurt.

In another preferred embodiment of the invention used for the thermalcracking of hydrocarbons, particularly of heavy oils, the carriersubstance is heated up in a first working space by burning thereincracking residues, as explained below. The hot carrier substance is thenpassed into a second working space, wherein the liquid hydrocarbons tobe cracked are atomized by means of steam, the resulting gaseous orvaporized cracking products being drawn from the second working spacefor subsequent cooling and further treatment, while the solid crackingresidues are discharged together with the carrier substance forrecirculation into the first working space.

BRIEF DESCRIPTION OF THE DRAWINGS The subject inventive process andapparatus or assembly will be better understood and other objects,advantages and features thereof will become apparent when attention isdirected to the following detailed description of preferred embodiments,such description making reference to the appended drawings, wherein:

FIG. I is a vertical section of an assembly comprising two workingspaces;

FIG. 2 is a vertical section of an assembly similar to that representedin FIG. 11, but with each working space divided in three sections;

FIG. 3 is an enlarged section of FIGS. 1 and 2, showing the porous wallsof the working spaces;

FIG. 4 is an enlarged section of FIGS. 1 and 2, with the walls of theworking spaces made of trellis tubes;

FIG. 5 is an enlarged vertical section of a working space with wallsconsisting of oblique steel plates arranged as in Venetian blinds;

FIG. 6 is an enlarged vertical section of a working space with wallsconsisting of ceramic plates, arranged like roof tiles;

FIG. 7 represents an alternative to FIG. 6 in a vertical halfsection;

FIG. 8 is a vertical section of an embodiment with annular crosssection;

FIG. 9 is a vertical longitudinal section of a refuse incinerating plantwith steam boiler, incorporating an embodiment of the invention forpreheating the combustion air;

FIG. 10 is a partial vertical section of an embodiment of the invention,used for drying sludge, with a stationary drying space;

FIG. 11 is a similar embodiment for drying sewage sludge,

but with a rotating drying space; and FIG. 12 is a longitudinal sectionof an assembly used for cracking light gasoline.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. I, the simplestembodiment of the invention is shown, with an upper working space l anda lower working space 2, both shaped as narrow pits in verticalarrangement. The sidewalls 3 and 4 of both working spaces are permeableto the streams of receiving and yielding agents, which flow through theworking spaces 1 and 2, entering and leaving in the direction of thearrows, through the ducts S and 6 and, respectively, 7 and 8. In bothworking spaces I and 2, the carrier substance is present as a thin layerfrom a hopper 9 and through a channel 110, and moves downwardly throughthe two working spaces in succession, the channel ll serving asconnection. The channel 12 leads into a cellular rotating block 13,serving for the control of the rate of recirculation, while a conveyor14 (not represented) ensures the recirculation as indicated by thearrow, the carrier substance being brought back into the hopper 9,thereby completing its closed circuit.

From the arrows in FIG. ll, it is apparent that both agents are flowingthrough the thin layer of carrier substance, crosswise to the verticalmoving direction of the latter.

When using the assembly described above for preheating air, hotcombustion gases are led through the duct 5 into the working space 1,therein to heat up the carrier substance by yielding at least part oftheir sensible heat, and then to leave through duct 7. The hot carriersubstance then moves through the channel 11 into the lower working space2, wherein fresh air is fed through duct 6. By flowing through the hotcarrier substance, this air is preheated and leaves as such through duct8. The rate of movement of the carrier substance through the workingspaces 1 and 2, which is that of its recirculation, is governed by thespeed of rotation of the cellular lock 13. A possible passage of airfrom the working space 2 into the working space I is prevented by thelength of the connecting channel 11, which is filled with the carriersubstance. Even at high air pressures in the working space 2, only verysmall amounts of air may surmount the resistance of the column ofcarrier substance in the channel II. The same is also true for thechannels 10 and 12.

In FIG. 1, the channels 10, II and 12 are shaped as vertical pits,coaxial with the working spaces I and 2. At higher working pressures,for instance in the working space 2, the cross section of the channels10, II and 12 may be reduced by means of internal partitions, equallyspaced, thereby opposing a higher resistance to the possible overflow ofan agent from one working space into another.

In FIG. 2, another embodiment is shown, also having two working spaces 1and 2 in vertical arrangement, the working spaces, however, beingdivided each into three sections 21, 22, 23 and respectively, 24, 25,26. The agent entering through the duct 27 into the working space 1, asindicated by the arrow, flows across the section 21 and is then ledthrough the duct 28 across the section 22 and through the duct 29 acrossthe section 23, to leave eventually through the duct 30.

The other agent follows an analogous path through the three ducts 31, 32and 33, to cross in three steps the lower working space 2 and leaveeventually through the duct 34.

The carrier substance is recirculated by means of a conveyor 35, intothe working space 1 through a hopper 36 and a channel 37 and movessuccessively through the upper working space 1, then channels 38 and 39,the lower working space 2 and the channel to a discharging device. Thisdevice essentially comprises a horizontal channel 41, wherein a plunger42 is moved toand-fro by a hydraulic drive, thereby propulsing thecarrier substance through the channel 43 into the recirculating conveyor35 (not represented). To prevent the overflow of any agent from oneworking space into the other, an occluding agent is led through the duct44 into a chamber 45, between the channels 38 and 39.

The permeable sidewalls of the working spaces in FIGS. 1 and 2,symbolized by dotted lines, may be of various structures andconfigurations to best ensure the free passage of the agents and thefree movement of the carrier substance.

FIG. 3 depicts an enlarged section of a pitlike working space, with twosidewalls 51 of porous structure, permeable to clean gases, and betweenthe sidewalls a thin layer 52 of a coarse-grained carrier substance, forinstance ceramic pebbles or metallic balls, easily sliding downwards andoffering little resistance to the agents through-flow.

FIG. 4 depicts a vertical section of a pitlike working space withsidewalls consisting of metallic trellis, convenient for a coarsecarrier substance 54. According to the operating temperatures and topossible chemical influences, suitable materials are used, for bOLl'Ithe trellis 53 and the carrier substance 54.

In FIG. 5, a vertical section through a pitlike working space ispartially represented. Here, the working space is limited by sidewallsconsisting of oblique steel plates 55, arranged as in Venetian blinds.For greater rigidity and for cooling, tubes 56 are welded to the plates55. In the two opposed sidewalls the plates 55 are displaced with regardto each other, thereby ensuring a homogeneous layer 57 of carriersubstance between them. The tubes 56 and the plates 55 may beincorporated in the heating surface of a steam boiler, for instance, asa vaporizer or as a superheater. in this embodiment with the sidewallsas described, both fine and coarse carrier substances may be used.

Similarly to FIG. 5, FIG. 6 shows an enlarged vertical section of aworking space with sidewalls consisting of ceramic plates 58, supportedby tubes 5%, as is the case with roof tiles. Lateral noses 60 protrudingat the lower ends of the plates 58 ensure equal spacing betweenconsecutive plates. Here again, the tubes 59 may be used as a heatingsurface in a steam boiler. The embodiment with ceramic plates 58 hasbeen found particularly suitable and is recommended for operation athigher temperatures.

In FIG. 7, an embodiment similar to that of FIG. 6 is disclosed, withthe tubes 61 having longitudinal fins 62, fitting into correspondinggrooves of the plates 63, for higher rigidity of the assembly. At veryhigh operating temperatures, the sidewalls may consist entirely ofceramic material, especially when heat transmission through thesupporting tubes is not possible or not desirable.

FIG. 8 depicts an embodiment for heating gases such as air for drying orfor combustion purposes, the embodiment being a self-supporting unitwith working spaces of annular cross section. In a gastight cylindricalcasing 101, preferably made of steel plate and lined with refractorybrickwork, two working spaces are located one above the other, eachcomprising two section 102-103 and, respectively, 104-105. These foursections are of annular cross section, with sidewalls 106 and 107 of thekind represented in FIGS. 5, 6 and 7, the carrier substance slidingdownwards from section to section. The carrier substance preferablyconsists of a refractory material, such as corundum, siliconcarbide andthe like, with a preferred granulation of 4-8 mm., and is fed throughthe connection 108. A conical device 109 ensures even repartition of thecarrier substance into the annular inlet opening 110.

Hot combustion gases, coming through the duct 111, enter into theannular space 112 and flow across the layerof carrier substance in thesection 102. Thereupon, the gases flow upwardly into the central chamber113, and then across the layer of carrier substance in the section 103,to enter into the annular chamber 114 and be eventually expelled throughthe outlet opening 115, after having transmitted their sensible heat tothe carrier substance in the sections 102 and 103.

The air to be heated is led through a duct 116 into an annular chamber117 and flows across the layer of carrier substance in the lower section104. The partially heated air thereupon flows upwardly into the centralchamber 118, to cross again the layer of carrier substance in the uppersection 105 and then to enter into an annular space 119, from where thehot air is expelled through the outlet opening 120.

The carrier substance moves downwardly, taking up heat in the sections103 and 102 and then yielding it in the sections 105 and 104, and iseventually discharged through the channels 121 evenly located around theannular periphery. For the passage of the carrier substance from theupper into the lower working space, several evenly distributed channels122 are provided In a similar way, connecting channels are providedbetween the sections 102 and 103 of the upper working space and 104 and105 of the lower working space.

As is apparent in FIG. 0, the layers of carrier substance in the twoworking spaces may be of different thickness, according to therespective requirements. In the upper sections 102 and 103, i.e. betweenthe sidewalls 106, a thickness of some 50 mm. is recommended, againstmm. in the sections 104 and 105, between the sidewalls 107.

Moreover, the possibility exists for different layer thicknesses to beadopted in the sections 102, 103 and 104, respectively.

FIG. 9 is a schematic representation of a refuse incinerating plant witha steam boiler for the recovery of heat, where the combustion air ispreheated by means of an assembly according to the invention, serving asa heat exchanger, the assembly being incorporated in the steam boiler.Combustion gases effluent from the combustion chamber 131 pass into afirst flueduct 132, the right-hand sidewall of which constitutes theworking space 133 of the heat exchanger. The two permeable sidewalls134, defining the working space 133, are of the kind represented inFIGS. 3, i or 5 anddescribed above. Through the tubes carrying theplates steam would be flowing for superheating. Above the working space133, a bunker 135 is provided for the carrier substance. A narrowchannel connects the first working space 133 to a second one 137, placedcoaxially under the first, with sidewalls 138 similar to 134, permeableto the stream of air to be preheated. A fan 139 drives the air through aduct 140 into a chamber 141, from where it flows across the carriersubstance in the working space 137 and then into a chamber 142, to beducted through 143 below the grates, as under-blast for the burning ofrefuse. The carrier substance passes from the lower working spacethrough a channel 144 into an intermediate bunker 145, from where it isrecirculated by an elevator M6 into the bunker 135.

When flowing across the carrier substance in the upper working space133, the combustion gases yield a part of their sensible heat to thecarrier substance, thereupon pass into a second flue-duct 147, wheretheir residual sensible heat is transmitted to the heating surfaces 148of the steam boiler. Eventually, the combustion gases pass through thededusting equipment 149 and the fan 1511 to be expelled through thestack 151.

in this embodiment, molten and granulated slag resulting from theincinerating process, in sizes of 3-6 mm. may be used as the carriersubstance. At practically no cost, this material is resistant to hightemperatures and to chemical action by the combustion gases as well asto superheating, which may occur during a standstill of the circulation,due to some disturbance.

A further important advantage lies in the fact that, in the case ofdust-loaded combustion gases, the performance of the carrier substanceis not hindered, and the dust retained in the carrier substance mayeasily be separated by sieving.

Mineral carrier substances, such as sand or metallic and ceramicsubstances, may of course also be used.

A further important advantage is that, because of the large crosssection for the flow of the combustion gases in the upper working space,the thickness of the layer therein may be kept at some 3040 mm., with adrop of pressure-head of only a few mm. of water column for the streamof combustion gases.

The arrangement described above may also be used when aggressive orpoisonous substances have to be removed from combustion or other gases.in such cases, carrier substances are used which are able to bindchemically such noxious substances as nitrous gas or combinationscontaining chlorine, fluorine or phosphorous.

For the drying of sewage sludge, metallic balls such as steel balls ofabout 25 mm. in diameter are used as the carrier substance. ln this casealso, the working space for the heating-up of the steel balls may beincorporated in a steam boiler as already described with respect to FIG.9.

FIG. depicts a schematic representation of a drying plant for sewagesludge. The first working space 133 for the heating up of the steelballs used as the carrier substance is located between the flue-ducts132 and 147 of the boiler, and are defined by the permeable sidewalls1341. The hot carrier substance moves from the working space 133 andthrough the cellular lock 161 into the second working space 162. Here,the carrier substance slowly slides over an inclined chute 163, whilethe liquid sludge is being sprayed over it by means of the nozzles 164fed through the pipes 165. In contact with the hot carrier substance,the sewage sludge is dried, leaving the working space 162 through thecellular lock 166 together with the carrier substance from which it isseparated by means of a vibrating sieve 167. The dry sludge is collectedin a receptacle 168 from which it is pneumatically conveyed into thefurnace.

The water vapor resulting from the drying process is drawn from theworking space 162 through a duct 169 and caused to comingle with hotcombustion gases for deodorization. A stream of exhaust gases from theboiler is led into the working space 162 through duct 170 for continuousscavenging of the working space.

From the vibrating sieve 167, the carrier substance is directed into anelevator 171, to be brought back into the working space 133 and thuscomplete the cycle.

With some sewage sludges, solid crusts may adhere to the steel ballsduring drying which, however, peel off as small shells dispersed in thecarrier substance and are gradually crumbled and ground into fine dustand eventually separated in the vibrating sieve 167 during the followingcycle, all without hindering the normal operation.

When such shells are very hard and subsist as such in normal operation,it is recommended that another embodiment of the invention be used aswill be described below with reference to 1 16. 11.

In FIG. 11, the hot carrier substance issuing from the cellular lock 161is led through the channel 181 into a revolving cylinder 182, whereinthe liquid sludge is sprayed by a nozzle 11%, fed through the pipe 183.By means of helicoidal ledges 185 internally welded in the cylinder, thecarrier substance is caused to travel along its axis. During this traveland because of the tumbling and reciprocal friction of the steel balls,the adhering crusts are detached and ground into fine dust, which isthereupon separated in the usual way. scavenging gas may be blown intothe cylinder through duct 186, to accelerate the effluence of the watervapor resulting from drying.

The two embodiments according to H68. 111) and 11 make it apparent that,in some cases and for better performance, it may be desirable to shapeand build the two working spaces differently. Furthermore, it ispossible to carry out other physical or chemical processes in one of theworking spaces, coupled to the process of heat exchange proper, as isthe case in the embodiment described in FIG. 1 1, where the process ofdrying is coupled with that of grinding the shells of dried sludge.

In FIG. 12, a plant for cracking light gasoline is schematicallydepicted with three working spaces in vertical arrangement. In thefirst, upper working space, the carrier substance is heated in twosections 201 and 202, by hot combustion gases issuing from a combustionchamber 203. Fuel oil is burned therein in a burner 204, the arrows 205and 206 symbolizing the feed of fuel oil and of combustion air. Thetemperature of the combustion gases in the chamber 203 is kept undercontrol by the addition of comparatively cool exhaust gas from theboiler, which is recirculated over duct 207. The heating gas is directedthrough the ducts 208 and 209 to flow across the sections 201 and 2112in succession and then into the heating chamber 211, receiving lightgasoline and steam through the pipes 212 and 213, respectively. Here,the gasoline is vaporized and mixed with steam, the mixture flowingthrough 214 to and across the second, intermediate working space, theflow path leading through the ducts 215, 219, 2211, and 221 across thesections 216, 217, and 218 in succession, during which the gasolinevapors are cracked into hydrocarbons of lower molecular weight.Depending on the heating temperature and the catalyst used, theresulting gaseous product may contain more or less saturated andunsaturated hydrocarbons.

When the unsaturated hydrocarbons, such as olefins are preponderant, thehot cracked gas is led through duct 222 into a cooler 223, therein to becooled by injection of atomized water as symbolized by the arrow 22 8,to prevent possible secondary reactions between the components of thegas.

With saturated hydrocarbons or with synthetic gas essentially containingcarbon monoxide and hydrogen, a supplementary heat exchanger (notrepresented in H6. 12) is provided between duct 221 and the cooler 223,for utilization of the gass sensible heat.

The cracked gas leaves the cooler 223 through a duct 225 for furthertreatment.

Because of the fact that, at its entry into the second intermediateworking space, the mixture of gasoline vapor and steam is alreadypreheated, the carrier substance will still be at a comparatively hightemperature when leaving this working space. Accordingly, the carriersubstance is led through the channel 226 into a third, inferior workingspace, therein to be cooled. Part of the exhaust gases effluent from theheating chamber 211 is therefore led through the duct 227, the fan 220and the duct 2311 into said inferior working space, divided in twosections 231 and 233, and across which the stream of gas is flowing, thepath of the gas stream leading over 232, 234 and 207 back to thecombustion chamber 203. The other part of the exhaust gas is expelledthrough the stack 235. When particularly high temperature is required inthe combustion chamber 203, the third working space is used as an airpreheater.

The carrier substance is discharged from the third working space throughthe channel 236, at a rate kept under control by means of a cellularlock 237. Through elevator 238 the carrier substance is then broughtinto a bunker 239, to reenter the cycle through the channel 2411. Partof the carrier substance in circulation may be withdrawn forregeneration through the channel 241, placed at the lowest level, and isreplaced by an equal quantity of fresh or regenerated carrier substancethrough the feeding channel 242.

The channels 240, 243, 246, 226 and 236 are of sufficient length toprevent the undesired leakage of the agents through them. At higherdifferences of pressure between the working spaces, an occluding agentmay be used, as already mentioned. With special provisions, the plantmay be operated at pressures above and below the atmospheric pressure.

The plant described above may be adapted for cracking gaseoushydrocarbons, such as natural, casing-head or refinery gas, or for theproduction of synthetic gas.

Heavy hydrocarbons such as fuel oils may be cracked in the plant by boththermal and thermal-catalytic processes. In such cases, ceramic pebblesare preferably used as the carrier substance and/or as a catalystsupport. The carrier substance enters into a first working space forheating. The liquid hydrocarbons are atomized by means of steam, to besprayed into a second working space, wherein they are cracked, more orless, depending upon the temperature of the carrier substance. Thegaseous cracking products are withdrawn for cooling and subsequenttreatment. The solid residues of the cracking process are dischargedtogether with the carrier substance from the second working space, andare preferably recirculated into the first working space where theyburn, thus contributing to the heating-up of the carrier substance.

In cases when it is required to modify the boiling margin of liquidhydrocarbons by means of moderate cracking, the hydrocarbons may bepumped as such, i.e. in the liquid state, across the layer of carriersubstance, previously heated to a convenient temperature, metalliccarrier substances being preferable for this purpose. The oil cokeadhering to the carrier substance is burned off in the first workingspace.

The plant, according to the invention, as described above, may be usedfor exothermal chemical processes as well, especially when a certainamount of heat has to be withdrawn continuously to keep the reactiontemperature at its optimum level.

For a better understanding of the various processes above described, inconjunction with the schematic representations of the respectiveembodiments, the drying of liquid sewage sludge in combination withrefuse incineration will be analyzed in the following, as anillustrative example of the inventive technique.

A refuse incinerating plant ,as represented in FIG. 9 and equipped withan assembly according to FIG. 10 for the drying of sewage sludge, ispresumably built for a town of -for instance-140,000 inhabitants. Theassumable daily amount of sludge for this town is about 19 tons of drysubstance, corresponding to some 95 miof liquid sludge with 80 percentwater contents. For drying this sludge down to water contents of only afew percent, some 2.2X10 kilocalories of heat are necessary, whichaccording to the invention, have to be supplied by the hot carriersubstance. The carrier substance consisting of steel balls of 25 mm.diameter is heated in a first working space, incorporated in the refuseincinerator, up to some 450 C. During drying, the carrier substanceyields part of its heat contents, leaving the drying space at atemperature of about 150 C., together with some 800 kg./h. of solid, drysludge. For this-some 5.3 tons of carrier substance must be recirculated10 times per hour. The dried sludge is separated from the carriersubstance by sieving andthen conveyed into the furnace, there to beincinerated. The water vaporresulting during drying is led into thestream of hot gases issuing from the combustion chamber. A volume ofeffluent gas of some 23,000 Nm /h consisting of combustion gases fromthe burning of refuse and of dry sludge, comingled with the water vaporfrom the drying process is then led through the upper working space, toflow across the layer of carrier substance therein at a velocity ofabout 1 m./s. Through this, the temperature of the gaseous mixture isreduced from 750 C. down to about 500 C. With a thickness of 50 mm. forthe layer of carrier substance, the drop of pressure-head for the gasstream is of only 2 mm. of water column. The residual heat contents ofthe effluent gases is recovered by means of the heatingsurface of theboiler.

In summary, the invention, as described above, is based on the followingphysical facts and the following reasoning and observations:

The rate of heat transfer to a layer of loose substance is determined bythe coefficient of heat transfer, by the specific surface of thesubstance, and by the volume of the layer. The specific surface is verygreat with loose substances, being largely dependent of theirgranulation. That is why, with layers of a loose substance, large heattransmitting surfaces may be obtained under least volumes, which is aprevailing advantage of this means of heat transfer. Similar conditionsalso occur in the processes for the transfer of matter.

It should be apparent that in the case of gaseous or liquid agents incounterflow with the carrier substance, the latter must be present in arather thick layer, to ensure a homogeneous flow through it, which isthe assumption for an effective transfer of heat. This thickness maycause a large drop of pressure-head in the stream flowing through thecarrier substance which, in most cases, is not acceptable.

In the case of a gaseous or liquid agent in counterflow with a movingmass of loose substance present as a column or layer, the drop ofpressure-head is a linear function of the length of path, i.e. theheight of the column or the thickness of the layer, and a squarefunction of the velocity of flow. Therefore, and assuming a constantvolume of loose substance, the velocity of flow and herewith the drop ofpressure-head may be reduced both by increasing the free intervals inthe loose substance and by reducingthe length of path, i.e. preferring aflat layer to a high column. I

In practice, however, it is hardly possible to achieve a homogeneousflow through a thin layer of large extension, this ultimately being themain reason why, -in spite of their advantages, heat exchanges of thetype with agents in counterflow with a circulated carrier substance,have not been widely used. Both factors: the large drop of pressure-headin the rather high columns required for reasons of efficiency and theimpossibility to achieve homogeneous counterflow in flat layers, areapparently the cause of the general misbelief in heat exchangers of thistype.

This problem is solved .in a simple way with the novel process accordingto the invention, by causing the receiving and yielding agents to flowthrough layers of moving carrier substance, crosswise to the directionof movement of the latter. Based on this principle, various embodimentsare possible, ensuring sufficiently thin layers of a convenient carriersubstance, with sufficient free intervals for the flow of the respectiveagent through it. Thus, apart from the advantages of efficient heattransfer in a minimum of space, the drop of pressure-head may be reducedto values of no practical importance, whereby new and wide opportunitiesof application are opened.

Although it is true that for an agent flowing crosswise through a movinglayer of loose carrier substances, the outlet temperature may, undersome circumstances not be homogeneous, but rather increase in thedirection of movement of the carrier substance, this is of littleimportance in most cases as explained in the description of FIG. 9. Insome cases, however, the efficiency of the heat exchange may be reduced,because of the increased outlet temperature of the yielding agent. Bydividing the working spaces into two or more sections, connected bymeans of outside channels, this inconvenience may be overcome. Thearrangement as described achieves a combination of crossand counterflow,by means of which the agents temperature at its ultimate outlet is nothigher as it would be in pure counterflow. This combination may beachieved in a simple manner, without constructional complications andincreased costs.

A salient fact is, that by the flow crosswise to the direction ofmovement of the carrier substance, it becomes possibleto provide aplurality of working spaces for the simultaneous performance ofdifferent processes therein.

The structure of the permeable sidewalls of the working spaces will bechosen in accordance with the nature of the carrier substance, theoperating temperature, and the physical properties of the agents flowingthrough them. Various materials of metallic, ceramic, and organic naturemay be used and the mostadequate form of execution may be adopted. Thus,even in the planning stage, optimum operating conditions may be ensured.

For the continuous and homogeneous discharge of the carrier substancefrom the last working space, mechanisms of contemporary and known typesmay be used. Depending upon the shape of the outlet and of the operatingconditions, cellular locks, cellular rolls, plungers or pusher-typeexpellers and so forth may be used. For the recirculation proper, i.e.the return of the carrier substance into the first working space, meansof known type may be adopted, such as continuous bucket elevators andthe like.

The examples of embodiments represented in the drawings are by far notexhaustive of all possibilities. The assembly, according to theinvention, may be used with exothermal chemical processes, particularlywhen a certain amount of heat has to be withdrawn continuously, so as tokeep the reaction temperature at its optimum level.

From the above considerations, it should be apparent that the inventionis not limited to the embodiments represented and described, but offerswide constructional and functional opportunities for adaptation to themost varied operational requirements as will be appreciated by thoseskilled in the art.

We claim:

1. A process for the drying of liquid sludge comprising the steps of:providing a solid carrier substance formed of metallic balls; moving thecarrier substance downwardly under the effect of gravity through anupper working space; passing combustion gases into the upper workingspace; heating the carrier substance by means of the combustion gases;passing the heated carrier substance through a lower working space;spraying sewage sludge at the lower working space upon the carriersubstance during continuous travel thereof, thereby causing watercontained in the sewage sludge to spontaneously vaporize; contacting thewater vapor collected at the lower working space with a stream of hotcombustion gases for deodorization; discharging the dried sludge fromthe lower working space together with the carrier substance; separatingthe dried sludge from the carrier substance; incinerating the separateddried sludge; and recirculating the carrier substance back into theupper working space.

2. A process as defined in claim 1, wherein the dried sludge isseparated from the carrier substance by sieving.

3. A process as defined in claim 1 including the step of altering thepath of movement of the sewage sludge during its travel through theworking space.

4. A process as defined in claim 1, wherein a revolving cylinder isprovided as the lower working space, the liquid sewage sludge beingsprayed on the hot carrier substance thereat, and wherein crusts ofdried sludge adhering to the metallic balls comprising the carriersubstance are peeled off during tumbling in the revolving cylinder.

2. A process as defined in claim 1, wherein the dried sludge is separated from the carrier substance by sieving.
 3. A process as defined in claim 1 including the step of altering the path of movement of the sewage sludge during its travel through the working space.
 4. A process as defined in claim 1, wherein a revolving cylinder is provided as the lower working space, the liquid sewage sludge being sprayed on the hot carrier substance thereat, and wherein crusts of dried sludge adhering to the metallic balls comprising the carrier substance are peeled off during tumbling in the revolving cylinder. 